Windows and Glazing

Introduction

Within This Page

Windows have long been used in buildings for daylighting and ventilation. Many studies have even shown that health, comfort, and productivity are improved due to well-ventilated indoor environments and access to natural light. However, windows also represent a major source of unwanted heat loss, discomfort, and condensation problems. In 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion (one-fourth of all the energy used for space heating and cooling).

In recent years, windows have undergone a technological revolution. High-performance, energy-efficient window and glazing systems are now available that can dramatically cut energy consumption and pollution sources: they have lower heat loss, less air leakage, and warmer window surfaces that improve comfort and minimize condensation. These high-performance windows feature double or triple glazing, specialized transparent coatings, insulating gas sandwiched between panes, and improved frames. All of these features reduce heat transfer, thereby cutting the energy lost through windows.

Description

Window systems are comprised of glass panes, structural frames, spacers, and sealants. In recent years, the variety of glass types, coatings, and frames available for use in window systems has increased dramatically, as has the opportunity to fine-tune and optimize window selection on a project-by-project basis.

For specific aesthetic and performance objectives the specifier may also wish to specify:

Tints (colors) and Coatings

U-Value

U-value indicates the rate of heat flow due to conduction, convection, and radiation through a window as a result of a temperature difference between the inside and outside. The higher the U-factor the more heat is transferred (lost) through the window in winter.

U-factors usually range from a high of 1.3 (for a typical aluminum frame single glazed window) to a low of around 0.2 (for a multi-paned, high-performance window with low-emissivity coatings and insulated frames).

A window with a U-factor of 0.6 will lose twice as much heat under the same conditions as one with a U-factor of 0.3.

Total (or net) window U-factors can be considerably higher than the center-of-glass U-factors.

Solar Heat Gain Coefficient (SHGC)

SHGC indicates how much of the sun's energy striking the window is transmitted through the window as heat. As the SHGC increases, the solar gain potential through a given window increases.

The SHGC is a ratio between 0 and 1. SHGC = 0 means none of the incident solar gain is transmitted through the window as heat and SHGC = 1 means all of the incident solar energy is transmitted through the window as heat.

A window with a SHGC of 0.6 will admit twice as much solar heat gain as one with a SHGC of 0.3.

The term "SHGC" is relatively new and is intended to replace the term "shading coefficient (SC)." While the terms are related, the shading coefficient of glass is defined as the ratio of the solar heat gain through a given glazing as compared to that of clear, 1/8 inch single pane glass.

Solar RadiationCourtesy of Efficient Windows Collaborative

Heat FlowCourtesy of Efficient Windows Collaborative

Visible Transmittance (Tvis-glass)

Tvis-glass indicates the percentage of the visible portion of the solar spectrum that is transmitted through a given glass product.

Sunlight is an electromagnetic form of energy exchange between the sun and the earth. It is composed of a range of electromagnetic wavelengths, generally categorized as ultraviolet (UV), visible, and infrared (IR) referred to collectively as the solar spectrum.

The short, UV wavelengths are largely invisible to the naked eye, but are responsible for fabric fading and skin damage. Visible light is made up of those wavelengths detectable by the human eye. This light contains about 47% of the energy in sunlight. Longer IR wavelengths are also invisible and contain about 46% of the energy in sunlight.

For a given glazing system, the term "Coolness index (Ke)," also called Efficacy Factor, is the ratio of the Tvis-glass to the shading coefficient (SC).

Tints (Color) and Coatings

The properties of a given glass can be altered by tinting or by applying various coatings or films to the glass.

Glass tints are generally the result of colorants added to the glass during production. Some tints are also produced by adhering colored films to the glass following production.

Tints are usually selected for aesthetic purposes. Some tints also help reduce solar gains.

Coatings, usually in the form of metal oxides, can also be applied to glass during production. Some of these coatings, called "low-emissivity" or "low-e," help reduce radiant heat transfer between panes of glass by blocking some or all of the IR wavelengths. These coatings can dramatically lower the window U-factor.

Care should be taken in specifying tints and coatings, as their application can dramatically impact window heat loss and heat gain. Mis-specification can result in the exact opposite of the desired performance.

From a performance perspective, specifying window U-factor, SHGC and Glass Visible Transmittance (Tvis-glass) means there is no need to specify tints and coatings.

There are far more glazings available for buildings today than there were even a few years ago.

Some recommendations for specifying windows and glazings include:

In general cases, specify low U-factors (< 0.40) for residential applications. Even lower values may be desired in extreme heating climates.

When specifying windows performance, take care to specify "whole product performance values" for U-factor and SHGC. Use of "glass-only" U-factors should be avoided as they can be 10% to 40% better than the whole product value.

For commercial buildings in conjunction with daylighting strategies, analyze the trade-offs between standard glazing and high coolness index (also called spectrally selective) glass. Spectrally selective glass has a relatively high visible transmittance and a relatively low SHGC.

In general, low SHGC windows should be considered for east- and west-facing glazing as a means of controlling solar heat gain and increasing occupant comfort. For large commercial and industrial structures, specify low SHGC windows on the east, south, and west facades. SHGC for north-facing windows is not critical for most latitudes in the continental United States.

For buildings where passive solar heating energy is desired, south-facing windows with high SHGC values coupled with low U-factors should be specified.

Select windows with comfort in mind. The proper specification of windows can result in higher Mean Radiant Temperature (MRT) in winter and lower MRT in summer, improving occupant comfort and productivity. MRT represents the average temperature an occupant feels from radiant heat exchange with their surroundings.

C. Other Attributes

Other important attributes of window and glazing systems include:

Gas Fills—Inert gases such as argon and krypton are often injected between panes of glass to reduce conductive and convective heat transfer. These low-cost, gas fills reduce U-values without affecting shading coefficients or visible transmittance.

Fritting—Baked on ceramic coatings, or frits, can be applied to the surface of glass in many different patterns, colors, and densities.

Safety and Security Glass—Information on the applications and benefits of laminated architectural glass can be found here. For specific information, visit the websites of the Manufacturers listed under Additional Resouces on this page.

International terminal building in Istanbul's Ataturk Airport uses laminated glass for safety and securityPhoto Courtesy of DuPont Laminated Technologies

Switchable Optics—A new generation of dynamic glazings are available that change optical properties through changes in light, temperature, or voltage (i.e., photochromic, thermochromic, electrochromic.) Currently, these materials are economically viable for niche markets only, such as two-way mirrors.

D. Opportunities and Cautions

Through its loan program, NYSERDA helped the Capital District Train Authority install new, energy-efficient glazing and high-efficiency heating and cooling equipment at this train station in Rensselaer, New York.Photo Courtesy of NYSERDA

Some design opportunities and cautions concerning the specification and application of window and glazing systems include:

Opportunities

Using high-performance windows can dramatically reduce heating and cooling loads, and eliminate the need for perimeter heating in internal-load dominated buildings due to the effect of increased Mean Radiant Temperature (MRT) on occupant comfort (see High-Performance HVAC).

Window systems with low-e and spectrally selective coatings can filter damaging UV wavelengths and increase the life of room furnishings.

Cautions

All low-e coatings are not the same! If selective coatings have been identified as a strategy to increase performance, make sure the coating is fully and properly specified for your application.

Always consider glare control, especially in commercial and industrial applications. Limiting contrast ratios and providing visual comfort in the field of view is critical, particularly in daylighting applications.

Avoid condensation problems. Condensation occurs when the glass surface temperature falls below the dew point of the room air. It can damage window and wall elements and obstruct views.

Historic buildings often require special window detailing. The desire to achieve historical accuracy can sometimes conflict with the desire to provide energy efficiency. Fortunately, several companies are now offering high-performance products that can replicate the appearance of historical windows while maintaining energy efficiency.

Application

Case Study

Argonne National Laboratory—Argonne, Illinois

The State of Iowa Facilities Improvement Corporation (SIFIC) and the Mental Health Institute in Independence, Iowa joined forces to identify and implement energy management improvements. Among several strategies, the team installed more than $300,000 worth of energy-efficient windows. To date, the Institute has saved more than $100,000 annually in energy costs.

Argonne National Laboratory, Argonne, Illinois, is one of DOE's first buildings to pursue LEED certification by the U.S. Green Building Council. The design includes more than 15 building materials chosen for their recycled, renewable, or lower-emitting content. In addition, several energy conservation features, such as high-performance windows selective to west and north orientations, will reduce electric consumption by 20% and natural gas by 30%, lowering the building's greenhouse gas impact by 55 tons/year. More

Design and Analysis Tools

Window Properties

WINDOW, a federally funded computer program developed by Lawrence Berkeley National Laboratory (LBNL), calculates U-values, SHGC, and Tvis of window systems constructed from glass and frames of known properties.

Window Design Strategies

Commercial and Institutional—To analyze the effect of different window areas and glass properties, investigate the following:

Computational Fluid Dynamics (CFD)—This advanced computer-based analysis method can help analyze the effects of complex fenestration systems on air currents and interior ventilation patterns. For more information: CFD Online

Others

About WBDG

WBDG is a gateway to up-to-date information on integrated 'whole building' design techniques and technologies. The goal of 'Whole Building' Design is to create a successful high-performance building by applying an integrated design and team approach to the project during the planning and programming phases.